1. Field of the Invention
This invention relates to an engine part, and more particularly to an engine part made of a combination of a ceramic member and a metallic member.
2. Description of the Prior Art
To improve the thermal efficiency of engines and improve the service life of various engine parts, it is effective to use an engine part which is made of a composite combination of a ceramic member and a metallic member, because such composite engine parts have a high heat insulation and a high wear resistance.
As an example of the engine part for improving the thermal efficiency of engines, a heat-insulating type piston utilizing ceramics has been contemplated. Two different structures for the piston utilizing ceramics have been proposed; namely, a structure made of a ceramic piston crown fastened to a piston body by using a metallic bolt, and a structure made by casting a metallic piston skirt on the outer circumferential surface of a ceramic piston crown.
In the case of the structure made of a ceramic crown fastened to a piston body by a metallic bolt, as disclosed in the specification of U.S. Pat. No. 4,242,948, the heat insulation of the piston is insufficient because the heat leaks to the piston skirt through the metallic bolt. Due to the need of the heat-resistance, the material of the bolt is generally different from that of the piston skirt, and the fastening at the bolt tends to be loosened by repetition of thermal expansions and shrinkages caused by the difference of thermal expansion between the bolt and the piston skirt. Besides, this type of structure has a shortcoming in that highly sophisticated machining techniques are necessary for boring a bolt hole in the ceramic piston crown.
In the case of the structure made by casting a metallic piston skirt on the outer circumferential surface of a ceramic piston crown, the coefficient of thermal expansion of the ceramic material of the piston crown is smaller than that of aluminum alloys or spheroidal graphite cast iron which are generally used for casting the piston skirt, so that the structure has a shortcoming in that the ceramics are susceptible to cracking due to the shrinkage of the cast metal during and after the cooling. To solve this shortcoming, U.S. Pat. No. 4,245,611 proposed a structure in which cushion layer made of fibrous ceramic material was disposed between the ceramic piston crown and the cast metal. Japanese Patent Laid-open Publication No. 122,659/81 proposed a structure in which a metallic ring is shrink fitted on the outer circumferential surface of the ceramics piston and a piston skirt is cast on the outer circumferential surface of the thus fitted metallic ring. However, the structure using the fibrous ceramic cushion layer has a shortcoming in that the bond between the piston crown and the piston skirt tends to be insufficient so that the piston crown is apt to be loosened during the operation of the piston. The structure using the metallic ring shrink fitted to the ceramic piston crown has a shortcoming in that high precision is required in the machining of the ceramic piston crown and the metallic ring.
Additionally, WIPO Publication No. 82/01034 proposes a tappet having a cam contact surface made of ceramics, for the purpose of improving the wear-resistance of engine parts. The tappet thus proposed has a shortcoming in that the ceramic member for the cam contact surface is shrink fitted on the tappet body, and high precision is required in the machining of the ceramic and metallic members thereof.